Chapter 6: Programming Fundamentals

4 + 4.5 + 0.375

8.875

1500 * 0.08875

133.125

995.50 * 0.08875

88.350625

def tax(subtotal):
    return subtotal * 0.08875

def return(subtotal):
return subtotal * 0.08875

  File "/tmp/ipykernel_3436/2480525607.py", line 1
    def return(subtotal):
             ^
SyntaxError: invalid syntax

tax()

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/1737278063.py in <module>
----> 1 tax()

TypeError: tax() missing 1 required positional argument: 'subtotal'

tax(1500)

133.125

def tax(1500)

  File "/tmp/ipykernel_3436/3862704308.py", line 1
    def tax(1500)
               ^
SyntaxError: invalid syntax

def tax(1500):
    return subtotal * 0.08875

  File "/tmp/ipykernel_3436/3680281193.py", line 1
    def tax(1500):
               ^
SyntaxError: invalid syntax

1500 + tax(1500)

1633.125

200 + 1500 + tax(1500)

1833.125

sum = 1500
sum = sum + sum * 0.08875 # Add the tax amount

sum

1633.125

def set_a():
    a = 10
    print('The value of a:', a)

set_a()

The value of a: 10

a = 20
print ('Initially, the value of a:', a)
set_a()
print('Finally, the value of a:', a)

Initially, the value of a: 20
The value of a: 10
Finally, the value of a: 20

def scoped(la, lala):
    lalala = lala + lala - la
    return lalala
first = 10
second = 11
third = 12
scoped(2, 4)

first, second, third replaced by la, lala, lalala

first

10

second

11

third

12

6.4 Iteration (Looping) Abstracts along Sequences

l = [7, 4, 2, 6]

this is called an assignment | numbers in bracket assigned to variable named l

for num in l: print (num)

7
4
2
6

for num in l:
    print(num)

7
4
2
6

NOTE: iteration template / pattern for iteration is

for in :

___

for num in l:
    print(num * 2)

14
8
4
12

def mean(sequence):
    for element in sequence: 
        total = total + element
    return total / len(sequence)

NOTE: remove the indent to one level to enter the last line / represents one code block ending

temperature - 80

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/3334237015.py in <module>
----> 1 temperature - 80

NameError: name 'temperature' is not defined

def mean(sequence):
    total = 0
    for element in sequence:
        total = total + element
    return total / len(sequence)

total = 0 means initializing the variable total / assigns the value 0 to total

short = [5, 5, 5, 5]

mean(short)

5.0

6.5 Polymorphisim Abstracts across Types

x = 5

type(x)

int

type(5)

int

x = 'hello world'
type(x)

str

str means string / sequence of characters AKA "hello world"

type(x)

str

Q: how to directly check the value of a string?

concatenation = the operation of putting two strings together by using +

'hello' + 'world'

'helloworld'

'knock' + 'knock'

'knockknock'

Assign the strings you're using to variables and then concatenate the variables:

a = 'hello'
b = 'world'
a + b

'helloworld'

Using len() to provide the length of an argument:

len('hello world')

11

len([1, 2, 3])

3

works for both strings and lists

• strings = returns how many characters are in the string
• lists = returns how many elements the list has

len(['hello', 'world'])

Q: what does the in the bracket on the left mean?*

NOTE:

• not everything has a length
• not everything can be added together

len(5)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/4108502181.py in <module>
----> 1 len(5)

TypeError: object of type 'int' has no len()

2 + 'hello'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/2556979404.py in <module>
----> 1 2 + 'hello'

TypeError: unsupported operand type(s) for +: 'int' and 'str'

NOTE:

• don't take the length of an integer
• don't add an integer to a string

Converting integers into strings

len(str(5))

1

• tells Python to treat integers as strings
• tells us how many characters are in the string

str(2) + 'hello'

'2hello'

^ tells Python unit '2' should be concatenated to 'hello'

• casting = converting between types
• polymorphism = when a single arithmetic operator/function/method is able to work on different types of data (integers/strings/lists)

Revisiting "Double, Double"

volume = [4.0, 2.0, 3.0, 5.5]
result = []
for element in volume:
    result = result + [element * 2]
result

[8.0, 4.0, 6.0, 11.0]

^ this code takes a list of volume levels and doubles it

What's happening in the line within the loop:

result = []
result

[]

^ the variable was assigned the value of an empty list

result = result + [4.0]
result

[4.0]

result = result + [2.0]
result

[4.0, 2.0]

^ the list is being built up AKA a list is being added/concatenated to the current result

result = []
result = result + 4.0

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/440397681.py in <module>
      1 result = []
----> 2 result = result + 4.0

TypeError: can only concatenate list (not "float") to list

what went wrong:

• [] = empty list
• 4.0 = float (number with a decimal point)

avoid DRY = use double() !

• ✓ bundled
• ✓ can be reused
• ✓ does iteration (via using a for loop)
• ✓ implements polymorphism | function can work on any element that can be multiplied by 2
• ✓ can double strings
• ✓ will work on sequences

Example of double() to double strings:

words = ['hello', 'world']

double(words)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/2412218211.py in <module>
----> 1 double(words)

NameError: name 'double' is not defined

Q: ^ how do I define 'double' again??

Example of a list with different types of elements (that can be multipled) | double() can take the list as an argument:

countdown = [3, 2, 1, 'contact']

double(countdown)

double('abstraction')

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/3286799117.py in <module>
----> 1 double('abstraction')

NameError: name 'double' is not defined

6.7 Testing Equality, True and False

2 + 2 == 4

True

Using = and == in Python:

• = for assignment (giving a variable a value)
• == to test if the left and right-hand sides are equal

2 + 2 == 3 + 1

True

2 + 2 == 2 + 1

False

NOTE: Boolean data type = value doesn't have quotation marks around it AKA True or False

NOTE: checking equality is most useful when there's at least one variable involved

Two-step equality test (to see what variables can be included):

greeting = 'hello'

(NOT A TEST): assigns a value to the variable "greeting" / nothing is returned

greeting == 'hello'

True

(A TEST): using the == operator

greeting == 'hi'

False

greeting == 'sawasdee'

False

greeting = '2'

greeting == '2'

True

greeting == 1 + 1

False

6.8 The Conditional, if

example 1:

def secret(word):
    if word == 'please':
        return 'Yes!'

example of test for equality using ==

secret('hello')

secret('world')

secret('please')

'Yes!'

a conversation with Python

• "secret('please') returns the value Yes! to signal that the correct word was input
• causes certain code to run ONLY if the condition is true
• example of the conditional statemet AKA if statement

secret('please')

'Yes!'

secret('Please')

Q: if nothing returned, does this mean I just redefined secret with another word? or does it mean that the incorrect word was input?

example 2:

def yesno(word):
    if word == 'yes':
        return True
    if word == 'no':
        return False

yesno('haha')

yesno('no')

False

yesno('yes')

True

yesno('Yes')

example 1.2:

if word == 'please':
    return 'Yes!'
else:
    return 'No.'

  File "/tmp/ipykernel_3436/617998124.py", line 2
    return 'Yes!'
    ^
SyntaxError: 'return' outside function

new keyword else = indicates indented code that should run otherwise (when the if condition doesn't hold)

example 1.3 (secret() to return True or False instead of two strings:

def secret(word):
    if word == 'please':
        return True
    else:
        return False

this can be refactored

• refactor = revise code to make it easier for people to read/understand/modify/further develop while keeping the function of the code the same
• ex: True when word has the value 'please' and False otherwise -> new version of secret() =

def secret(word):
    return (word == 'please')

return whether or not the value of word is the string please (so we can return the result of that equality test)

6.9 Division, a Special Error, and Types

don't ever allow your program to attempt to divide by zero

Strings can be multiplied by integers:

'hi' * 2

'hihi'

Strings cannot be divided by integers:

'hihi' / 2

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_3436/3445710781.py in <module>
----> 1 'hihi' / 2

TypeError: unsupported operand type(s) for /: 'str' and 'int'

[Exercise 6-1] Half of Each:

half()